1. Field of the Invention
The application relates to a method utilized in a wireless communication system, and more particularly, to a method of handling service rejection for circuit switch service in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
In the LTE system, a UE performs a service request procedure for transferring the EMM mode from EMM-IDLE to EMM-CONNECTED mode and establish the radio and S1 bearers when uplink user data or signalling is to be sent. Note that, a UE is in EMM-IDLE mode when no NAS signalling connection between UE and network exists, and a UE is in EMM-CONNECTED mode when a NAS signalling connection between UE and network is established. Another purpose of the service request procedure is to invoke mobile originating (MO)/mobile terminating (MT) CS fallback or 1xCS fallback procedures.
Moreover, if the UE is configured for NAS signalling low priority, and the last received ATTACH ACCEPT message or TRACKING AREA UPDATE ACCEPT message from the network indicated that the network supports use of EXTENDED SERVICE REQUEST for packet services, the UE shall send an EXTENDED SERVICE REQUEST message with service type set to “packet services via S1”. If the last received ATTACH ACCEPT message or TRACKING AREA UPDATE ACCEPT message from the network did not indicate that the network supports use of EXTENDED SERVICE REQUEST for packet services, the UE shall instead send a SERVICE REQUEST message. After sending the SERVICE REQUEST message or EXTENDED SERVICE REQUEST message with service type set to “packet services via S1”, the UE shall start T3417 and enter the state EMM-SERVICE-REQUEST-INITIATED. Note that, a UE enters the state EMM-SERVICE-REQUEST-INITIATED after it has started the service request procedure and is waiting for a response from the MME.
However, the service request procedure may be not accepted by the network. If the service request cannot be accepted, the network returns a SERVICE REJECT message to the UE including an appropriate EPS Mobility Management (EMM) cause value. When the EMM cause value is #39 “CS service temporarily not available”, the MME shall include a value for timer T3442 in the SERVICE REJECT message. On the other hand, if a mobile terminating CS fallback call is aborted by the network during call establishment, the MME shall include the EMM cause value #39 “CS service temporarily not available” and set the value of timer T3442 to zero.
In addition, if the timer T3442 value received in the SERVICE REJECT message is not zero, the UE shall start timer T3442. On the other hand, if the T3442 value received in the SERVICE REJECT message is zero, the UE shall not start timer T3442. Note that, the UE shall not try to send an EXTENDED SERVICE REQUEST message for mobile originating services to the network, except for mobile originating CS fallback for emergency calls, until timer T3442 expires or the UE sends a TRACKING AREA UPDATE REQUEST message.
As abovementioned, the UE may uses EXTENDED SERVICE REQUEST message for packet services if it has been indicated that the network supports use of EXTENDED SERVICE REQUEST for packet services in the ATTACH ACCEPT message or TRACKING AREA UPDATE ACCEPT message. Nevertheless, the applicant notices a problem associated to SERVICE REJECT with cause of CS service temporarily not available. In detail, the UE is not allowed to send EXTENDED SERVICE REQUEST message when the timer T3442 is running. In other words, the UE is not allowed to initiate mobile originating service during this time when the network rejects the UE with EMM cause #39 (CS service temporarily not available) with timer T3442. This will impact the mobile originated data service as the UE is not allowed to send EXTENDED SERVICE REQUEST message for mobile originated data service while timer T3442 is running. However, the UE shall be allowed to use PS service even though the CS service is not available. Therefore, the abovementioned behavior causes mobile originated data service delay.